This invention relates to water-soluble, ternary cellulose ethers.
It is well known that the presence of hydroxyethoxyl substitution increases the hydrophilic character of cellulose ethers. Unfortunately, however, the presence of such hydroxyethoxyl substitution also generally raises the gel or cloud point of the cellulose ether. For example, water-soluble methylcellulose ethers generally exhibit cloud points in the range from about 40.degree.-70.degree. C. However, the addition of hydroxyethoxyl substitution thereto often increases the gel point of the resulting hydroxyethylmethylcellulose above the boiling point of water. Similarly, in U.S. Pat. No. 3,873,518 a water-soluble, ternary hydroxyethylhydroxypropylmethylcellulose ether (HEHPMC) is disclosed having high hydroxyethyl and hydroxypropyl contents. The cloud points of these ternary ethers are in excess of 70.degree. C. and are often above 80.degree. C. By contrast, corresponding hydroxypropylmethylcellulose ethers having similar hydroxypropoxyl and methoxyl content as those HEHPMCs exhibit cloud points in the range from about 50.degree.-65.degree. C. Thus it is seen that the addition of hydroxyethoxyl substitution substantially increases the cloud point of the cellulose ether.
In many applications it would be desirable to have a cellulose ether having the advantages of hydroxyethoxyl substitution which also exhibits a cloud point comparable with those methylcellulose and hydroxypropylmethylcellulose (HPMC). For example, in suspension polymerization processes, water-soluble cellulose ethers, such as HPMC and methylcellulose are employed as protective colloids. The utility of such cellulose ethers in such polymerization processes depends on their insolubility in water above temperatures in the range from about 50.degree.-70.degree. C. A hydroxyethoxyl substituted cellulose ether having a cloud point within such a range would be useful as a protective colloid in suspension polymerization processes.
Another characteristic of conventional cellulose ethers which exhibit a gel point such as methylcellulose and hydroxypropylmethylcellulose is that the temperature at which they precipitate in solutions of water upon heating is much higher than the temperature at which they redissolve upon cooling. In many conventional cellulose ethers, the difference between the cloud point (i.e., the temperature at which it becomes insoluble upon heating in aqueous solution thereof) and the return cloud point (i.e., the temperature at which the cellulose ether redissolves upon cooling) is in the range from about 15.degree.-35.degree. C. The existance of this so-called "hysteresis loop" often presents disadvantages in the use of these cellulose ethers. For example, it is often desirable to dissolve cellulose ethers in water by dispersing cellulose ether in hot water and then cooling the dispersion below the return cloud point of the cellulose ether. Due to the hysteresis loop, it is necessary to cool the dispersion far below the nominal cloud point of the cellulose ether. Often, the cooled solution must then be reheated somewhat to be suitable for its intended use. Reduction of this hysteresis loop would eliminate the need for excess cooling and reheating of such solutions, and would result in a reduction of time and energy necessary to use cellulose ether solutions.
Accordingly, a cellulose ether having hydroxyethoxyl substitution which has a cloud point comparable to conventional methylcellulose and HPMC ethers would be desired. In addition, a cellulose ether having hydroxyethoxyl substitution which exhibits a significantly reduced hysteresis loop as compared to most conventional cellulose ethers would also be highly desirable.